Mysterious brake trouble (again)

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Mysterious brake trouble (again)

Postby Hotdog » Thu Dec 06, 2007 11:35 am

The power of the front brake of the Comfy Chair has faded away to nothing and I'm at a loss as to the cause. The brakes in question are Avid BB5 mechanical disc brakes. These have an adjustable static inner brake pad and a mobile outer pad, with barrel adjusters at both ends of the cable. The calliper unit is aligned with the rotor by loosening the cunning self adjusting mount bolts, holding the brake on and tightening the bolts.

So, the possible causes of trouble that I think I've eliminated are:

1. Pad adjustment - both inner and outer are nice and close, the pads clamp the rotor with plenty of brake lever movement to spare.
2. Warped rotor - I've trued it, it looks fine.
3. Calliper alignment - the rotor is centred in the calliper and the calliper and rotor appear parallel.
4. Glazed/contaminated brake pads - I tried a light sanding of the surface with some clean, fine sandpaper but it didn't seem to help.
5. Brake pad wear - the pads are fairly worn down, but there's still a good covering of pad material left.

The identical rear brake, that I adjust and maintain in the same way, is working great. I'm running out of ideas here, short of swapping the front and rear callipers to see what difference that makes. One problem I had before due to the pads being worn down was the calliper itself (not the brake lever) running out of travel, when that happened it felt like the rotor was being firmly clamped when in fact the calliper was hitting its end stop and only gently squeezing the rotor. I solved that by realigning the calliper nearer to the mobile pad. I don't think that's the problem now though.

Any ideas?
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by BNA » Thu Dec 06, 2007 11:53 am

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Postby Kalgrm » Thu Dec 06, 2007 11:53 am

Has the loss of braking power come on gradually or suddenly? If gradual, it could be cable stretch. If sudden, it could be contaminated pads.

I have now bought Goodridge sintered pads from Chain Reaction for both my MTB and my 'bent. In both cases, they were much better than the original pads (Hayes on the MTB, Avid on the 'bent). I can highly recommend them if you are considering changing the pads.

Cheers,
Graeme
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Postby MichaelB » Thu Dec 06, 2007 11:59 am

Typically with automotive brake pads (& tyres for that matter), as there is les & less material, it doesn't work as well due to age, hardening etc.

I know there is less heat in a bike disc brake than a car, but maybe it's just that the pads are worn and the linings are not the same coefficient of friction that they used to be ?

What % are they worn ?

Either try new ones, or a cheap test will be to swap front & rear ?

Have you deglazed the disc ? Light rub with wet & dry ?

Are the pads worn straight ?
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Postby europa » Thu Dec 06, 2007 11:59 am

Graeme's post has triggered a possibly incorrect memory of something I read on Sheldon Brown's site where he blamed most braking problems on the cable.

So - rusty cable causing binding, a kink in the system somewhere, maybe you moved something and put a dud bend in the system, a knock on the cable putting a dent in the outer, small demon in the brake caliper pushing the pads apart as you're trying to squeeze them together.

Things to look for anyway.

Richard
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Postby Hotdog » Thu Dec 06, 2007 12:28 pm

Loss of braking power has been fairly rapid, though I don't think it was instantaneous so it's not a definite pointer to contamination. With surface contamination or glazed pads I'd have expeced the light going over with wet and dry I gave them to have helped at least a bit, and it doesn't seem to have done. The rotors look pretty clean, and I gave them a gentle wipe with a soft cloth anyway.

The pads are now quite worn and could do with replacement soon anyway so I will be ordering some new ones in. Thanks for the Goodridge tip Graeme, I'll look into those. The pads are worn at a bit of an angle, which is down to the BB5 calliper mechanism, it's not that sophisticated and the cable actuated 'pistons' don't push the pads quite straight. I don't think that's a problem, though, once they're bedded in.

The cables seem fine, free and smooth movement and any stretch will have been taken up by me adjusting the mobile pad position.

It may be that my pads are simply worn out, or it could be a recurrance of the calliper limit problem (I'll do a proper check for that this evening). It's a bit of a confusing one this as everything looks like it's working, I squeeze the brake lever and the cable moves, the calliper actuates and the pads appear to grip the rotor. All is great, except there's sod all friction being generated :?
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Postby Hotdog » Thu Dec 06, 2007 2:56 pm

Kalgrm wrote:I have now bought Goodridge sintered pads from Chain Reaction for both my MTB and my 'bent. In both cases, they were much better than the original pads (Hayes on the MTB, Avid on the 'bent). I can highly recommend them if you are considering changing the pads.

Unfortunately it seems the Goodridge (and most other 3rd party manufacturer's) pads are only available for all Avid disc brakes except the BB 5s (BB 7 mechanicals, Juicy 5 and Juicy 7 hydraulics all use the same pads but the BB 5 pads are a different shape for some reason). For my BB 5s I only seem to have a choice between Avid original pads or Kool Stop 3rd party, both organic rather than ceramic or sintered. I've ordered some new Avid pads for now.

At some point I may feel an urge to upgrade to BB 7s, there are a number of differences that I can imagine making a fair bit of difference.
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Postby Kalgrm » Thu Dec 06, 2007 4:10 pm

BB7s are indeed good. I didn't even bother installing the BB5s which came with the bike.

Try swapping front and rear pads in the short term.

Cheers,
Graeme
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Postby Mulger bill » Thu Dec 06, 2007 9:20 pm

I suspect pad contamination...

Course the fixed pad might need a tweak.

What about the cable attachment to the lever, some levers run different points for Vs or discs.

Shaun
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Postby Hotdog » Fri Dec 07, 2007 8:45 am

I have figured it out, essentially what was happening was that the calliper was running out of throw just after the pads had contacted the rotor. This made it look like all was well to a casual inspection, but the effect was that the pads were only in gentle contact no matter how hard you squeezed the brake lever. The problem arises because of the design of the BB5 brakes, and is caused by pad wear.

Unlike the BB7s (and I believe most other disc brakes) there is no outer pad adjustment on the BB5s. As the pads wear down you need to adjust their position to maintain the small gap to the rotor. The inner (static) pad has an adustment knob to do this, but to move the outer (mobile) pad you need to tighten the brake cable thereby partially actuating the calliper. The problem is that once there's been a significant amount of brake pad wear you can end up using all the calliper throw just to get the outer pad into position, leaving no movement left for actual braking. There is a workaround to this, you can reposition the calliper closer to the wheel so you need to use less calliper throw to position the outer pad relative to the rotor. There is a limit, however, because moving the calliper means you need to use more inner pad adjustment which will eventually run out of throw too. At that point you simply have to replace the brake pads. I now understand why Avid say that the pads must me replaced when the total thickness (including backing material) is less than 3mm. It's not the amount of braking material left that matters, it's whether the pads are too small for the BB5's limited capacity for adjustment.

Anyway, I've already got some new pads on the way which will make everything wonderful again. In the meantime I've been able to use my new found insight to get my front brakes working, though I had to reposition the calliper so that I'm using all of the inner pad adjustment and I'm still using a fair bit of calliper throw to position the outer pad. Definitely time for those new pads.
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Postby europa » Fri Dec 07, 2007 9:07 am

You know, I'm really not looking forward to having disc brakes on my bent. I know they are more powerful and are better in the wet and all that guff, but considering I find V brakes and even caliper brakes plenty good enough for my purposes, disc brakes are just too damned fussy. There are a dozen threads on here talking about the messing about they need. Great if you need the extra power or in a situation where rim brakes could cause problems, but for normal riding?

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Postby Hotdog » Fri Dec 07, 2007 10:22 am

europa wrote:You know, I'm really not looking forward to having disc brakes on my bent. I know they are more powerful and are better in the wet and all that guff, but considering I find V brakes and even caliper brakes plenty good enough for my purposes, disc brakes are just too damned fussy. There are a dozen threads on here talking about the messing about they need. Great if you need the extra power or in a situation where rim brakes could cause problems, but for normal riding?


I'm not sure that disc brakes in general are fussier than rim brakes, they're just different and a little less intuitively obvious in their operation and adjustment. When people are new to them (as I am) it takes a while before they've got them all figured out, but once you have they'd don't need much fiddling with. I seem to remember having plenty of trouble getting V-brakes properly set up and aligned too, way back when I was new to them (and bike maintenance in general). As far as routine maintenance is concerned it's mostly just a case of turning the knob to keep the pads close to but not touching the rotor, really no different than keeping rim brake pads close to but not touching the wheel rim by using barrel adjusters. In my case the fact that my cheap brakes lack the outboard knob complicates things, but not impossibly.

Recumbents do benefit especially from disc brakes, in my opinion. As they're aerodynamically slippery at all times (can't sit up to deploy the 'air brakes') you're more reliant on the brakes. This means that during a long descent under braking heat build up would be more of a problem for a rim braked 'bent than an upright, but that's not an issue with discs. Likewise, rapid deceleration from high speed needs more braking power on a 'bent than an upright as aerodynamic drag helps out less. Another way of looking at it is that 'bents can make full use of more powerful brakes than uprights can, the lower centre of gravity means that you can decelerate harder before the rear wheel lifts. So for a 'bent the extra stopping power of disc brakes isn't just overkill, you can really use it if you have to.
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Postby europa » Fri Dec 07, 2007 11:30 am

Yeah, just me fretting because the rotten thing is on the docks in Perth, not under my bum teaching me about disc brakes 8)

Richard
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Postby Hotdog » Fri Dec 07, 2007 11:39 am

europa wrote:Yeah, just me fretting because the rotten thing is on the docks in Perth, not under my bum teaching me about disc brakes 8)

Ooh, it's getting closer then. Do you have an ETA for it yet?
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Postby europa » Fri Dec 07, 2007 12:01 pm

Not as such. Apparently Craig won't have it in his hands until the end of next week. I've asked both him and Graeme to give it a good test ride first - I want to know whether the problems I'm having are due to the bike or due to me being an utter novice.

Craig is planning to get her to me before Christmas. I'm sure that if that doesn't happen, it won't be through lack of trying :D

And it's got the same disc brakes that you have and Graeme rejected ... unless the factory was out of stock and they fitted Tektros instead :roll: Hell, I don't care, as long as both wheels touch the ground at the same time :wink:

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Postby McPete » Fri Dec 07, 2007 5:11 pm

Hey Richard, there might be a set of Avid Juicy 3 hydos coming on the market later this year at a very reasonable price :P
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Postby Kalgrm » Fri Dec 07, 2007 5:44 pm

Hey Pete,

Keep me in mind too - my HFX-9 calipers on the Epic will need replacing soon, since I can't swap the in-line for the bleed line. :(

Cheers,
Graeme
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Postby Mulger bill » Fri Dec 07, 2007 10:09 pm

Adjustment issues like this is what made me upgrade Princess to hydros, I haven't had to touch them except for cleaning and an occasional caliper realign. Mind you, the pads (Juicy7s) are looking a bit thin, I'll give 'em fresh fluid and a bleed then.

Shaun
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